This invention relates to a method for producing refractories, for example, fire-bricks, and particularly to a method for producing lining bricks of a blast or steel-making furnace to be used in areas contacting molten steels.
Among these refractories have been known are those which contained a variety of carbonaceous materials as a binder or an impregnant. That is to say, coal tar pitch has been employed as a binder or an impregnant heretofore. However, since it contains a large amount of, substances detrimental to the humane body such as a nitrogen-containing compound or oxygen-containing compound, there occur sanitary problems for operators during the production or use of the refractories. Further, it is necessary to dilute the coal tar pitch with anthracene oil or creosote oil in order to facilitate the workability during the manufacturing process of refractories. As a result, the toxicity of impregnants is largely increased owing to the incorporation of of such a solvent. Accordingly, the present situation is that a harmless substitute for coal tar pitch is strongly desired.
In addition, as described in Japanese Patent Publication Laid Open No. 51-115512, it has been known that asphalt obtained by deasphalting of petroleum, or a product obtained by heating a petroleum asphalt to 350.degree.-450.degree. C. is employed as an impregnant or a binder. However, when a refractories using an asphalt-containing binder are practically employed, for example, as a lining brick for a convertor, under the severe conditions during its use the bonding force is very weak in comparison with that of coal tar pitch, and therefore, its effect to prevent the permiation of slag thereinto is so low that it can not be put into practice.
There is also disclosed the use of substances as an impregnant or a binder which are obtained by heating and polymerizing a heavy oil obtained by steam cracking of petroleum in the presence or absence of catalysts (Japanese Patent Publications Laid Open No. 51-10811, No. 51-10816, No. 51-114409). Although the above defects resulting from use of coal tar pitch are eliminated by these methods, in the case the substance obtained by one step heat treatment of the cracked heavy oil in the presence or absence of catalyst is employed as an impregnant or binder for refractories, it is not fully satisfactory to the improvement of slag-resistant properties of refractories and to the decrease of loss thereof, as shown later in Comparative Examples.
In addition to this, when the heavy oil disclosed in these methods is heat treated in one step, carbonaceous material is deposited in reactors for heat treatment. As a result, it becomes impossible to prevent local heating, and therefore, heat-treated products can not be obtained continuously on an industrially advantageous scale. This makes it impossible to produce refractories industrially and conveniently.
On the other hand, in the case where it is intended to obtained a useful pitch by heat treatment in one step, there occur the following defects.
Pitch used for a binder of refractories is desired to have a high content of fixed carbon (Conradson carbon) from the problem on its coking property, and is required further to have a softening point ranging from 60.degree. to 100.degree. C. from the viewpoint of workability. Such a pitch as is employed for impregnating refractories is also desired to have a high content of fixed carbon (Conradson carbon) owing to the problem on its slag-resistant property and is required to be liquid at room temperatures (about 20.degree. C.) or to have a softening point of less than 100.degree. C. from the view point of workability. However, at the same time when a highly fixed carbon-containing pitch is obtained by a simple heat treatment in one step of a heavy residium produced as a by-product in the above thermal or steam cracking, its softening point increases making it difficult to use it as binder. For example, if the residium is heat treated to obtain a pitch containing fixed carbon of more than 50%, the softening point of the product pitch inevitably increases to about 150.degree. C. and intended pitch can not be obtained.
For example, in Japanese Patent Publication No. 46-2417, a heavy residual oil is heat treated to obtain a pitch having a boiling point of more than about 400.degree. C., however the softening point of pitch obtained is as high as 230.degree.-250.degree. C.
Even in the case of heat treatment in two steps, as disclosed in Japanese Patent Publication Laid Open No. 48-73045, the softening point of pitch obtained is as very high as 120.degree.-215.degree. C., wherein a heavy residual oil is treated in the first step under the conditions of a pressure of 20-200 Kg/cm.sup.2, a temperature of 400.degree.-600.degree. C. and a time of 10-1200 secs., followed by removing of a lighter oil boiling at a temperature below 400.degree. C., and in the third step is treated by heating under the conditions of a lower temperature of 300.degree.-480.degree. C., a pressure of from atmospheric pressure to 50 Kg/cm.sup.2 and a time of 1-10 hrs.
As a means for adjusting softening points of pitch, it is suggested to control the given softening points thereof by adding a lighter oil such as gas oil or a heavy oil to a pitch with high softening point (Japanese Patent Publication No. 43-30073). However, in the case that these hydrocarbon oils are included in the pitch, they are gasified on its calcining. As the result, a molded body such as refractories is expanded rapidly to bring about the change of shape of product, the formation of hollows and further the problem of rupture in the product. Accordingly, it is impossible to obtain a satisfactory molding.
Further, in Japanese Patent Publication Laid Open No. 49-35420, a heavy oil is heat treated at a temperature of 350.degree.-470.degree. C. under pressure, and a lighter component or oil is eliminated to control the softening point, if necessary. That is to say, the softening point can be varied depending upon the amount of lighter oil to be removed and the manner of its removal. However, the presence of any lighter component in the product pitch would prevent one from obtaining a satisfactory molded body, as described before. A pitch of practical use does not contain substantially fractions boiling at temperatures lower than 400.degree. C. The softening point of pitch obtained from a heavy oil having a boiling point higher than 400.degree. C. is remarkably increased, as disclosed in Japanese Patent Publication No. 48-73405.